1. Field of the Invention
The present invention relates in general to optoelectronic devices (such as light-emitting diodes (LEDs) or semiconductor lasers made of compound semiconductor materials) and a method for manufacturing the devices. More particularly, the present invention relates to semiconductor light-emitting devices made of gallium nitride (GaN) based semiconductor material and a method for manufacturing the same. Such devices essential for developing full color displays and, in the case of coherent light sources, high density optical storage technologies. Also, the devices are very likely to be used as devices for signal and illumination applications.
2. Description of the Prior Art
The GaN based semiconductors such as In.sub.x Al.sub.y Ga.sub.1-x-y N show high-efficiency radiative recombinations due to their direct transition natures. And, in view of the available wide range of bandgaps from 2.0 eV to 6.2 eV, the GaN based semiconductors have been developed as materials for high-efficiency light-emitting devices such as short wavelength semiconductor lasers and high-brightness short wavelength LEDs, which are active in the green, green-blue, blue, and ultra-violet (UV) spectral regions.
In.sub.x Al.sub.y Ga.sub.1-x-y N, a quarternary semiconductor, is basically combined of nitride based (N-based) binary semiconductors such as GaN, aluminum nitride (AlN), and indium nitride (InN). Among the N-based compound semiconductors, GaN has been widely employed as a material of optoelectronic devices. However, GaN has a rather high melting point of 1700.degree. C. or higher as well as an extremely high equilibrium vapor pressure of nitrogen at the growth temperature, so that its stoichiometric composition (stoichiometry) is hard to control and therefore is difficult to grow bulk single-crystals. Presently, therefore, its single-crystalline growth technique mainly employs the halide vapor-phase epitaxy (HVPE) method. Recently the growth techniques that employ the metalorganic chemical vapor deposition (MOCVD) method in particular have greatly been developed, thus permitting ternary crystals such as In.sub.x Ga.sub.1-x N or Al.sub.y Ga.sub.1-y N to be obtained by mixing In or Al into GaN. By using a heterojunction that combines a plurality of such III-V nitrides, the luminous efficiency can be improved. By forming a double heterojunction (DH) structure, where the active region is bounded on both sides by higher bandgap materials, effective particularly in the confining of injected carriers and optical radiations, it is possible to provide high-brightness short-wavelength LEDs and short-wavelength semiconductor lasers.
To manufacture current-injection type light-emitting devices, which are based on a pn junction, it is important to control the electrical properties, so as to form desired p-type and n-type semiconductor regions. In GaN based semiconductors, the n-type doping can be relatively easily controlled by using Si as n dopant atoms. The hole concentrations in the p-type semiconductor regions, however, are generally hard to control and concerted efforts were made to dope GaN based semiconductor p-type. This is probably because that magnesium (Mg) and zinc (Zn), which are used as a main acceptor impurity for p-type semiconductor regions, have deep energy levels (large binding energies) and so low activation ratios (of 10.sup.-2 -10.sup.-3). Further, the growth by MOCVD method causes a passivation of acceptors by atomic hydrogen (H), which is a decomposition product of ammonia (NH.sub.3) used as a source gas (J. A. Van Vechten, et al. Jpn. J. Appl. Phys. 31 (1992) 3662).
That is, one known problem is that in the growth of a Mg-doped GaN layer by the MOCVD method etc., when the substrate temperature is lowered down to the room temperature (during which an NH.sub.3 gas generally continues to be supplied as a source of the column V element to avoid dissociation of N from the surface of the growth layer), the growth layer incorporates atomic H, which passivate the acceptors, thus providing a high resistivity of the Mg-doped GaN layer. In the case of a GaN layer in which Mg is doped as much as 1.times.10.sup.20 cm.sup.-3, its H concentration becomes 5.times.10.sup.19 cm.sup.-3, which generally results in 10 times or more as much as the case of an undoped and n-type GaN layers grown under the same conditions in terms of atomic H incorporation.
To overcome this problem, it has been found that the activation ratio can be improved by performing low energy electron beam irradiation (LEEBI) (H. Amano, et al. Jpn. J. Appl. Phys. 28 (1989) L2112). Subsequently it was discovered that thermal annealing at 700.degree. C. under N.sub.2 ambient can serve the same purpose as LEEBI process (S. Nakamura, et al. Jpn. J. Appl. Phys. 31 (1992) 1258), thus giving a possibility of realizing high-efficiency light-emitting devices. These methods, however, suffer in a respect that a complicated process of electron-beam irradiation or thermal annealing must be added in processing. Moreover, the process of LEEBI requires a complicated and expensive electron-beam irradiation equipment, increasing the cost of manufacturing.
Moreover, high-efficiency LEDs require high light extraction efficiency for their realization. Hence, it is necessary to provide such a special structure as used in the LEDs made of Ga.sub.x Al.sub.1-x As or GaP. This structure specifically attempts to increase the angle of cone of emission by growing a thick transparent layer with a low electrical resistivity. This structure is used to expand an internal light-emitting region, so as to take the light out of the device without being shielded by the electrode. With the GaN based materials, especially p-type ones, however, it is difficult to obtain crystals with a low resistivity. Moreover, the surface morphology deterioration due to the heavy doping becomes significant by making the transparent layer thicker. To improve the light extraction efficiency for GaN based LEDs, therefore, another method is proposed to form a current-blocked structure using an oxide film in order to prevent carrier injections in the light shielded area; this method, however, suffers in a respect that a process of oxide film deposition must be added, complicating the manufacturing processing.
In view of the foregoing discussion, to achieve a higher luminous performance of the conventional GaN based light emitting devices, it has been necessary to perform such complicated manufacturing processes as the LEEBI process and the thermal annealing for activating the acceptors or the special deposition process for forming oxide films. Those complicated processes and addition of new processes have adverse effects on the reproducibility and the reliability of the device characteristics, thus giving significant problems such as increased product costs and deteriorated yield.